Xiqun Lu

A mixed-lubrication model considering elastoplastic contact for a piston ring and application to a ring pack

A mixed-lubrication model considering the oil supply was developed. The elastoplastic model was applied in the asperity contact simulation. The oil-film thickness at the ring–cylinder liner interface was determined using a mass conservation algorithm. The modeling results were compared with experimental results for verification on a reciprocating wear tester. It is found that, under the fully flooded condition, the results of the Greenwood–Trip model and the current model in the middle of the stroke are in accordance with the experimental results, but the results of the current model at the ends of the stroke are closer to the experimental results than the Greenwood–Trip model results are. Under the starved-lubrication condition, the friction coefficient of the current method is closer to the experimental result than that of the Greenwood–Trip model in the entire stroke. Therefore, the model developed in this study is appropriate for mixed lubrication under the fully flooded condition and the starved-lubrication condition. Furthermore, the model was applied to the ring pack of a diesel engine to study the effect of the oil supply on the tribological performance. The results show that the frictional forces under the 1 μm oil supply condition are far larger than those under the sufficient oil supply condition in the middle of the stroke, and so the increase in the frictional force may be used to identify scuffing failure in future studies if the relationships between the frictional forces and the measured vibration signals of the engine are obtained.

Numerical modeling of lubrication of piston ring of two-stroke marine diesel engine considering the effect of multi-scale grooves on the cylinder liner

The method of lubrication for piston ring in some types of low-speed two-stroke marine diesel engines is completely different from those in medium- or high-speed diesel engines since the inner surface of cylinder liners are textured with elliptical grooves in macroscale and honing texture in microscale. In this paper, a numerical model has been developed to study the potential of use of cylinder bore surface texturing, in the form of circumferential oil grooves with different dimensions and densities, to improve the tribological properties of piston ring–cylinder liner tribosystem. The average Reynolds equation has been employed in the area of microscale texture and the effect of macroscale grooves has been incorporated to improve the currently lubrication model. The focus is put on cavitation formation condition and boundary condition in grooves within the area of lubrication, analysis of the effect of macrogroove dimension characters and distribution of grooves on the tribological performance of piston compression ring. The solution provides the hydrodynamic friction force, boundary friction force, coefficient of friction, and the total friction force. The results show that the cylinder liner oil grooves can efficiently be used to maintain hydrodynamic effect. It is also shown that optimum surface texturing may substantially reduce the friction losses between piston ring and cylinder liner.

Effect of Cylinder Liner Oil Grooves Shape on Two-Stroke Marine Diesel Engine's Piston Ring Friction Force:

The dimensions, area densities, and geometry of macroscale surface textures may affect the performance of hydrodynamic lubrication interface. Reported in this paper are the investigations of the effect of surface textures bottom shapes on the friction forces between piston ring and cylinder liner for two-stroke marine diesel engine, using numerically generated textures and average Reynolds equation. These textures are on the cylinder liner surface in the form of circumferential oil grooves with different aspect ratios and different area densities. The hydrodynamic pressure distribution is also calculated using Reynolds boundary condition. The results revealed that the bottom shape could positively affect the friction between moving surfaces, as it could provide a microwedge or microstep bearing that tends to enhance the lubrication condition between piston ring and cylinder liner.

Interring gas dynamic analysis of piston in a diesel engine considering the thermal effect

Understanding the interaction between ring dynamics and gas transport in ring pack systems is crucial and needs to be imperatively studied. The present work features detailed interring gas dynamics of piston ring pack behavior in internal combustion engines. The model is developed for a ring pack with four rings. The dynamics of ring pack are simulated. Due to the fact that small changes in geometry of the grooves and lands would have a significant impact on the interring gas dynamics, the thermal deformation of piston has been considered during the ring pack motion analysis in this study. In order to get the temperature distribution of piston head more quickly and accurately, an efficient method utilizing the concept of inverse heat conduction is presented. Moreover, a sensitive analysis based on the analysis of partial regression coefficients is presented to investigate the effect of groove parameters on blowby.

Thermomechanical Fatigue Life Prediction for a Marine Diesel Engine Piston considering Ring Dynamics

A newly designed marine diesel engine piston was modeled using a precise finite element analysis (FEA). The high cycle fatigue (HCF) safety factor prediction procedure designed in this study incorporated lubrication, thermal, and structure analysis. The piston ring dynamics calculation determined the predicted thickness of lubrication oil film. The film thickness influenced the calculated magnitude of the heat transfer coefficient (HTC) used in the thermal loads analysis. Moreover, the gas pressure of ring lands and ring grooves used in mechanical analysis is predicted based on the piston ring dynamics model.

Online measurement of torsional stiffness and fault analysis for flexible coupling under working condition

In this paper, a method of measuring the relative torsional angles of the flexible coupling under working condition was developed. A double-encoder measurement system was proposed to get the torsional angles. The torsional angles were divided into “constant torsional angle” and “alternating torsional angle”. The measured results were analyzed further, which were applied to a series of flexible couplings. Firstly, the torsional stiffness of D-type flexible coupling was obtained according to the relationship between torques and torsional angles. The proposed method overcomes the difficulty that the angular displacements of flexible coupling cannot be measured online under operating condition. The feasibility and accuracy of the method developed in this study was verified by comparing stiffness curve with unloading curve obtained from static torsional experiment and values from factory inspection report. Furthermore, in view of the tear fault of A-type flexible coupling in a diesel generator set, the maximum alternating torsional angles of A-type, B-type, and C-type flexible couplings were measured under working condition, then safety margins were obtained for the diesel generator set. According to the safety margins of the three kinds of couplings, C-type flexible coupling is chosen to replace A-type, which provided the basis for type selection of flexible coupling. Finally, the method developed in this study would also be applied to the operation monitoring and fault diagnosis of flexible coupling online.

Numerical Analysis and Experimental Evaluation of Cylinder Liner Macro-Scale Surface Texturing

An experimental and theoretical study is presented to study the effect of surface texturing in the form of circumferential oil grooves on improving the tribological properties of piston ring-cylinder liner tribosystem. Tests were performed on a reciprocating test rig with actual piston rings and cylinder liner segments, and a numerical model has been developed. A comparison was made between the performance of the textured cylinder liners and un-textured cylinder liners. It was found that with the smaller oil groove area density, the reduction in friction force is more obvious, Parabolic and triangular oil grooves are more efficient in friction reducing, and the prediction results by numerical model match the experimental results well in most case.© 2015 ASME

Tribological effect of piston ring pack on the crankshaft torsional vibration of diesel engine

In this article, a mixed lubrication model considering the oil supply quantity and a friction model under dry running condition were developed, in which an elasto-plastic contact model was employed in calculation of asperity contact forces and friction forces instead of the assumption of pure elastic contact. This model was verified to be suitable for calculating the friction forces of piston ring under different conditions on a reciprocating wear tester. The friction forces of piston ring pack were considered as an excitation source to calculate the crankshaft torsional vibration besides the exciting torques of gas pressure in the combustion chamber and inertia forces of reciprocation components. Furthermore, an experiment on a refitted single-cylinder air compressor was conducted to validate the change rule of torsional amplitudes under normal and scuffing failure conditions. The results showed that the friction forces between piston rings and cylinder liner in the mid-strokes under starved lubrication and dry running conditions increase obviously compared with those under fully flooded lubrication condition. It is reasonable to ignore the exciting torques due to friction forces of piston ring pack for calculating crankshaft torsional vibration under normal condition, and the torsional amplitudes at 2.0 order frequency increase obviously under slight and serious scuffing failure conditions. The result of experiment on a refitted single-cylinder air compressor also agrees with the above conclusion about torsional vibration. So, the change in torsional amplitude at 2.0 order frequency can be used as a referenced rule to identify scuffing failure for diesel engine through analyzing the signals of torsional vibration.

Interfacial Stress and Failure Analysis for Piston Ring Coatings under Dry Running Condition

A two-dimensional thermomechanical finite element model was developed to analyze the sliding process of a piston ring with coating sliding on cylinder liner under dry running condition. Thermal and mechanical effects were considered simultaneously in the model. The aim of the current work is to study the mechanisms of scuffing, failure, and seizure occurrence in a piston ring-liner system. It is shown that coating thickness plays an important role in the thermal and mechanical stress status at the contact area, coating bulk body, and interface of the coating and piston ring substrate. The coating thickness also exhibits a significant influence on the temperature rising at the contact area and interface of the ring coating and substrate, which could cause failure at the interface of the coating and substrate before it happens at the contact surface under some specific conditions. The results also show that thinner coating thickness in some specific range could have a higher possibility of cracking or failure. Furthermore, it is found that the thermal loading is the key cause of scuffing or failure of the piston ring coating.

Analysis of Tribological Performance of Piston Ring Lubrication

In this paper a mixed lubrication model considering lubricant supply conditions on cylinder bore has been developed for the piston ring lubrication. The numerical procedures of both fully flooded and starved lubrication were included in the model. The lubrication equations and boundary conditions at the end of strokes were discussed in detail. The effects of piston ring design parameters, such as ring face profile and ring tension, on oil film thickness, friction force and power loss under fully flooded and starved lubrication conditions due to available lubricant supply on cylinder bore were studied. The simulation results show that the oil available in the inlet region of the oil film is important to the piston ring friction power loss. With different ring face crown heights and tensions, the changes of oil film thickness and friction force were apparent under fully flooded lubrication, but almost no changes were found under starved lubrication except at the end of a stroke. In addition, the oil film thickness and friction force were affected evidently by the ring face profile offsets under both fully flooded and starved lubrication conditions, and the offset towards the combustion chamber made a large contribution to forming thicker oil film during the expansion stroke. So under different lubricant supply conditions on the cylinder bore, the ring profile and tension need to be adjusted to reduce the friction and power loss. Moreover, the effects of lubricant viscosity, surface composite roughness, and engine operating speed on friction force and power loss were also discussed.Copyright